The other definition here is correct to a certain extent but it is really much more than that. It stresses individuality and freedom of choice but it also stresses an individuals responsibility to the choices he makes. Also discards all religion as a means to control human behavior. Existentialism is still a current movement but its most influential members came in the early 1900's. They include Sartre, Camus and Dostoevsky.

Existentialism is essentially about having to make choices. Every action and every non-action is the result of a choice to do, or not to do, something. The summation of those decisions - to act or not to act - is you - who you are.

"I was only following orders". Not so, you have the choice to follow or not to follow the orders. Only YOU can make the choice.. Your life is a succession of decisions. And those decisions create who you are.

But that's no bloody reason to stop trying. In struggle humans are beautiful creatures. We are each our own invention fighting against the waves of Entropy to give our life meaning. And THAT is why life is worth living.

The philosophic exercise of likening oneself to God in that the direction and meaning of your life is yours alone to determine. The admission to oneself that anything and everything you know to be true may in fact be false, and that you do not possess the knowledge to determine what is and isn't. The method of continually constructing personal arguments for your decisions despite the knowledge that what you do may or may not matter.

These are two conversations about existentialism.
"Do we exist solely as figments of the imagination of God?"
"Perhaps. Then is it possible that the characters of our own dreams and imagination are of a similar nature to ourselves?"

"Life is meaningless. I should skip work in the morning to drink and enjoy myself."
"No, perhaps you should be responsible. You never know when you might find out the reason why."
"True."

Ho Hum... Well this is a really tricky subject as most Existential and continental philosopher's would disagree with each other, and even then they would disagree that they disagree… and I don’t know why!
Nietzsche would say God is no longer important in our moral reasoning (Understand that the “God is Dead” Stuff isn’t a atheist point) Kierkegaard would totally disagree and say that the connection with God is what confines us to a true moral code. From what I understand is that they argue that we cannot be confined by prior groups or assumptions. That freedom and the morality of the choice of freedom we take is paramount. They hold Hegelian’s, Kantian’s and Marxist thinkers as the scourge of the earth but then theirs Sartre who was a bit of a Marxist so I don’t know…. and on a lighter note I would say Nietzsche though not anti-Semitic was a little scary in his moral reasoning, all that stuff to do with charity… even republicans wouldn’t hold that to be true.